American Elm – Ulmus Americana
‘Brandon’ Elm – Ulmus Americana ‘Brandon’ (shorter than U. americana and more broad-headed crown)
‘Patmore’ Elm – Ulmus Americana ‘Patmore’ (tall, upright vase-shape. Suitable for smaller sites)
American elm is my favorite shade tree for my region. Rated at zone 2, it is the toughest shade tree that will not only survive, but thrive in our area. American elm has a distinctive vase-shaped crown, very hard, dense wood, and a thick leafy crown. Despite its attractive form, however, it has a few potential problems. In Southern Alberta, European elm scale is a big problem, and has left many elms dead, and thousands are in dire health. Dutch elm disease (DED) has killed countless thousands of trees in North America, and while we haven’t had to deal with it in Alberta, it may be inevitable that it shows up. The scale problem is so bad in Calgary, that when people ask me about the threat of DED, I usually tell them not to worry, because the elm scale will likely finish off our elms before Dutch elm disease even gets here! I have been reluctant to plant new elms for the past few years, but have decided to once again use them in landscaping. We have effective treatments to manage European elm scale and DED, and I’m prepared to use those treatments on a regular basis for the pleasure of having healthy, majestic elms in our urban landscapes.
When to Prune
October 1st to March 31st, to prevent the spread of Dutch elm disease.
Structure and Pruning
Elm is one of a few species that, left to its own devices, often develops poor structure. Proper structural training after planting and for the first several years is important, if not critical, to develop strong branch attachments. I recently planted a #15 potted elm in my front yard. It needed about 4 small pruning cuts after planting to start the process of proper training. With elm, the main goal is to develop a strong central leader. We accomplish this by subordinating – reducing the length of – co-dominant branches. This sounds fancy but really its just a matter of being aware of two competing leaders and either removing one or cutting one back so that the tree recognizes the uncut branch as the true leader of the tree. This is a technique that has not been in use long enough for us to see the results on most mature elms. The majority of street tree elms we see today have major co-dominance issues, and as a result have developed structural problems, such as included bark, at the branch attachments. A properly pruned elm will become a strong mature tree that will require little maintenance and will resist storm damage.
Most often, I get the call to work on elms once they are mature, and they have either not had any pruning, or very poor pruning, over the years. If they have had no pruning, there is little I can do to change the structure of a mature elm. All I can do is manage the appearance of the tree by removing dead and diseased wood, and improve its safety by reducing poorly attached or over-extended limbs. Due to structural problems, it is often necessary to install bracing systems into mature elms, such as bolts and synthetic cables, to prevent branch failure in severe weather.
Health and Appearance
Remove dead, diseased, and damaged wood from the crown periodically to keep the elm looking healthy. On newly planted trees, I always install a mulched area around the tree to prevent competition from weeds and grass. Generally, as my specimen shade tree gets larger, I extend the mulched area each year or two, and may then begin to under-plant the tree with a shrub or two, or perhaps some perennials. Water is important, and a mulched bed is the most important thing any tree-owner can do to conserve moisture in the root-zone and prevent drought stress to the tree. Fertilization is unnecessary unless the tree shows specific nutrient deficiencies.
Many people express concern that their elm is too tall and are worried about their safety should the tree fail. The most common homeowner suggestion is that we top the tree and make it smaller. As with most trees, and shade trees in particular, topping is inappropriate and will lead to much bigger problems later. If the tree has poor structure and is getting very large, we can assess the tree as a potential hazard. All it may need is some reduction pruning to properly shorten a few over-extended or poorly attached limbs. Or, we may need to install a bolt and cable.
Elm tolerates hard pruning and I regularly restore trees that were mutilated by topping. On occasion, I perform a crown reduction every two to three years and completely reshape the tree. This practice is very high maintenance and costly, only recommended if the tree was topped, but is still highly valued in the yard and removal is not an option.
Roots and Foundations
The belief that roots attack foundations is largely a myth, especially on the Prairies where soil moisture is low and most of the roots grow in the top few inches of soil. That being said, I wouldn’t consider planting a large shade tree within 20-25 feet of the foundation, because studies have shown that in cases where roots have affected foundations, the majority of the cases occurred within 25 feet of the house. But the main reason to plant a large tree far from the dwelling is so it has room to grow without interference or the need for radical pruning as it gets older.
(c) 2014 Shane LePage, Wild Rose Garden & Tree Service Inc., Red Deer, AB.